Smuggled nuclear weapons are an urgent threat to the US and to all nations. Although radioactive weapons and their components emit gamma rays and neutrons, they are difficult to detect when shielded or obfuscated by clutter. An advanced particle detector is needed to reveal and localize threat materials rapidly and reliably. Localization data is also needed to increase the statistical power of each radiation scan in the presence of backgrounds, since even a few gamma rays or neutrons coming from a particular region of the cargo would reveal a hidden source, whereas non-directional detectors require hundreds or thousands of additional detections above background to raise a suspicion that some kind of source might be somewhere nearby. With source localization, the entire inspection process could be speeded up, reducing inspection times and entry waits at shipping ports. Clean loads could be cleared more quickly. Secondary inspections, when necessary, could use the location information as a starting point.
Some attempted solutions at directional detection include “gamma cameras” involving collimators such as pinhole or multi-channel collimators, or coded-aperture masks. Such detectors are notoriously inefficient since most of the gamma rays are absorbed in the collimator. Other attempted solutions use paired or elongate detector elements, which generally provide low efficiency and poor angular resolution, and require time-consuming iterative rotations to find the source. Even lower efficiency is characteristic of double-scattering type detectors that rely on measuring two Compton scatterings for directional gamma ray detection, or two proton-recoil scatterings for neutrons.
What is needed is a compact, rugged, efficient detector that indicates the specific direction of the source of gamma rays or neutrons without extensive searching or iteration. Preferably the new detector would have sufficient sensitivity to detect even well-shielded nuclear weapons, and sufficient angular precision to localize the source among clutter and obfuscation, rapidly, with high efficiency, and at low cost.